Not Applicable.
The invention relates generally to a new process for the treatment of biosolids resulting from the treatment of biological wastewater streams. More particularly, the invention comprises an autothermal aerobic process for treating biosolids where the temperature is controlled by adjusting the amount of shear generated through jet aeration devices. The invention provides for a truly aerobic environment under which thermophilic microorganisms will thrive. The invention also relates to a method and apparatus for controlling foam generated in a treatment reactor.
Wastewater such as sewage streams generally have various naturally occurring and/or man-made contaminants, notably organic contaminants. In a remarkable display of the versatility of nature, some naturally occurring microorganisms have the ability to consume these contaminants for their own life processes, thereby turning what is an undesirable pollutant into (for their purposes) a beneficial nutrient or food source. The wastewater treatment industry frequently capitalizes on the ability of these microorganisms by using such microorganisms in facilities that treat wastewater streams to destroy the contaminants and break them down into basic compounds. Wastewater streams are fed into tanks or ponds that maintain conditions conducive to microorganism activity. Typically, the microorganisms which consume the targeted contaminants are mesophilic and thrive at temperatures in the range of about 25 to about 50 degrees Celsius.
The desired result of this type of wastewater treatment is the destruction of organic contaminants, but a by-product of this type of treatment is the production or increase of a biomass or biosolids comprised of the microorganisms. The biosolids yield from waste water treatment can range from about 0.1 pound of biosolids per pound of biological oxygen demand (BOD) removed to about 1 pound of bacteria per pound of BOD removed. A more typical range of biosolids yield is from about 0.3 pounds to about 0.6 pounds of bacteria per pound of BOD removed. Disposal of this biosolids may still be problematic, even after many contaminants have been consumed by microorganisms. One problem arises from the pathogenic nature of many microorganisms, such as the Fecal Coliform group of organisms; although such microorganisms have proven beneficial in consuming contaminants, they themselves may pose a danger to human health and are disease causing organisms. These include but are not limited to certain bacteria, protozoa, viruses and viable helminth ova. Regulations by states and/or the federal government impose restrictions upon land disposal of materials containing pathogenic microorganisms. It is desirable to treat biosolids so that one can easily and legally dispose of the biosolids on land or under ground. Suitably treated biosolids may even prove to have beneficial uses. Under certain circumstances, it may be used as a soil conditioner or fertilizer.
Another problem with the biosolids may arise from the sheer volume of biomass generated. Costs associated with the production and disposal of biosolids include both capital costs and operating expenses, such as biosolids disposal costs, trucking costs, material handling costs, management costs, and liability costs associated with disposal. Most if not all of these costs depend on the volume of biosolids at issue, and a reduction in the amount of biosolids can make an economically unfeasible operation into a profitable one. Methods which will reduce the mass and/or volume of biosolids to be disposed have significant commercial and environmental benefits.
Biosolids also contains other materials including microorganisms which are not pathogenic in nature. Typically the biosolids includes a group of microorganisms that thrive in what is generally referred to as the thermophilic temperature range. These thermophilic microorganisms are normally not harmful to humans, and there are both aerobic and anaerobic bacteria that thrive within the thermpophilic range. This invention is especially interested in the aerobic microorganisms. Although the temperature ranges for classification of bacteria varies somewhat depending upon who is describing the range, thermophilic activity usually takes place within the range of from about 45xc2x0 C. to about 70xc2x0 C. In contrast, pathogenic bacteria usually thrive within what is referred to as a mesophilic range which is from about 25xc2x0 C. to about 37xc2x0 C. or the normal body temperature of humans, and may begin to die at about 38xc2x0 C.
Therefore, various methods have been proposed and practiced for treating the biosolids that results from treatment of wastewaters. Biosolids may be treated aerobically or anaerobically, with different microorganisms, conditions and results. Among the methods of biosolids treatment is autothermal thermophilic aerobic digestion (xe2x80x9cATADxe2x80x9d). ATAD capitalizes on the presence of materials in the biosolids such as naturally occurring microorganisms which are not pathogenic or harmful to humans but which will kill pathogenic microorganisms. Typically, these are thermophilic microorganisms which thrive at temperatures of from about 45xc2x0 C. to about 70xc2x0 C.
A preferred temperature for thermophilic microorganisms is approximately 65xc2x0 C. When this preferred temperature is maintained during the treatment of a wastewater biosolids, the reaction time for destruction of mesophilic microorganisms at 65xc2x0 centigrade for purposes of meeting governmental regulations is approximately three quarters of an hour, as established by the Environmental Protection Agency""s Standards for Use and Disposal of Sewage Biosolids, 40 CFR, Part 503. Three hours is an easily obtained processing time for most biosolids treatment facilities, since biosolids is often pumped once every twenty four hours from the waste water treatment plant.
In a typical ATAD process, biosolids resulting from wastewater treatment is treated in a reactor, which operates at a temperature in the thermophilic range, i.e., from about 45xc2x0 C. to about 70xc2x0 C. Temperatures above the above this range do not allow the thermophilic microorganisms to thrive and may even result in their destruction. Within this temperature range, thermophilic microorganisms are active in an aerobic process where they consume oxygen, which must be provided in the solution.
An advantage of an aerobic process using thermophilic microorganisms is that their use of oxygen is an exothermic reaction. The heat released as a result of this reaction raises the temperature of the biosolids solution. As the temperature rises above the mesophilic range, mesophilic microorganisms are killed and consumed by thermophilic microorganisms. It has been estimated by mothers that 9000 BTUs may be released for every pound of volatile suspended solids destroyed. The interrelated cycle processes in which exothermic reactions trigger additional exothermic activity by thermophilic microorganisms results in an autothermal process and thereby creates an autothermal environment by virtue of the maintenance of relatively high temperatures.
Pathogens could also be destroyed through the direct application of heat from an outside heat source to the biosolids solution. By directly heating the biosolids to temperatures that are inhospitable for mesophilic microorganisms, these pathogens may be killed. However, this type of treatment (without the action of thermophilic microorganisms) is costly and wastes energy, since the amount of heat that must be directly applied to raise the temperature of the biosolids mass is substantial.
A major challenge in operating an aerobic biosolids treatment process is to keep the process sufficiently aerobic by meeting or exceeding the oxygen demand while operating at the elevated temperatures in which thermophilic bacteria thrive. One reason why this is difficult is that as the process temperature increases, the saturation value of the residual dissolved oxygen decreases. That is, a higher temperature results in less oxygen remaining in the biosolids solution. Another reason is that the activity of thermophilic microorganisms increases with higher temperature. This higher activity results in increased oxygen consumption by the microorganisms. Hence, greater amounts of oxygen must be imparted to the biosolids solution.
Another major challenge is to operate the process in an autothermal condition while still maintaining some control over the operating temperature. In an autothermal process, the process operates at a temperature higher than ambient without adding heat or by adding less heat than would ordinarily be needed to maintain the process at that temperature. In the biosolids treatment industry, autothermal processes capitalize on the exothermic nature of the action of the thermophilic bacteria in breaking down and consuming mesophilic bacteria or other organic compounds. The use of autothermal processes can obviate the need for external heat supply. However, it is still desirable or necessary to have some means of controlling the temperature of the process.
The need to control temperature has been previously identified and discussed in U.S. Pat. No. 5,587,081, which discloses a method of controlling temperature by varying the proportion of fresh air versus recycled air injected into the biosolids. By increasing the amount of fresh cool air introduced, the reactor is cooled. However the inventor believes it is important to use fresh air in the injection process because recycled air is not as effective in providing oxygen for thermophilic bacteria to thrive. The process described in U.S. Pat. No. 5,587,081 does not appear to take into account the fact that recycled air, although warmer than fresh air, has less oxygen and will generate less exothermic reaction and heat from the thermophilic microorganisms. The recycled air has a lower content of oxygen than is found in ambient air. This results in less oxygen being imparted to the biosolids solution by the recycled air. Although at first glance, it may appear that the effect of the reduced oxygen content is minimal because the reduction in oxygen may be only a few percent, in practice the reduced oxygen content results in insufficient oxygen being imparted to the solution to create a truly aerobic environment for the aerobic microorganisms to thrive.
Various apparatus and methods have been used to inject an oxygen containing gas stream into a biosolids solution. For example, spargers, diffusers and aerators of various designs and configurations have been used. It is the inventor""s opinion that the best apparatus to deliver the necessary oxygen is the aeration jet. One such aeration jet has been developed by Mass Transfer Systems, Inc., (xe2x80x9cMTSxe2x80x9d) 100 Waldron Road, Fall River, Mass. MTS has been purchased by Waterlink and have been put under its biological wastewater systems division, which lists its address as 630 Currant Road, Fall River, Mass., USA 02720. A product brochure by MTS is enclosed herein and incorporated by reference. By using the aeration jet, it is possible to create finer air bubbles along with higher shear which results in greater introduction of oxygen into the biosolids solution. There are many other advantages associated with the aeration jet, including better mixing. As the biosolids treatment occurs and mesophilic bacteria are broken down, carbon dioxide, water and ammonia (as well as other organic compounds) are produced when the protoplasm within the cell is released into the biosolids solution. The ammonia raises the pH of the solution and causes a noxious odor. Additionally, cell breakdown results in foam. It is desirable to have some means to treat odor and foam.
A typical method of controlling foam has comprised breaking the walls of the foam bubbles by manual or physical means. For example, some reactors have employed one or more cutting blades rotated by a motor. The blades spin through the foam layer, thereby rupturing foam bubbles, converting the foam back into a liquid. There are disadvantages to this approach for controlling foam, including maintenance and energy costs and efforts, particularly for a high rpm motor. Furthermore, the cutting blades may erode over time and require periodic replacement. Another disadvantage is that the motor that rotates the cutting blades is typically placed at the top of the reactor (outside the biosolids solution and the foam). However, the heat that can build up at the top of the reactor may shorten the life expectancy of the motor.
The inventive process has been referred to by its inventor as the THERMAER(trademark) Process. The invention provides a method for controlling the temperature of an autothermal process by adjusting the flow rate(s) through a jet aeration nozzle of circulated biosolids solution and/or oxygen-containing gas, thereby adjusting the rate of exothermic reaction from the interaction of oxygen with aerobic thermophilic microorganisms. The mechanism by which the biosolids flow rate and/or gas flow rate affects the reaction rate is through the amount of shear produced as the biosolids solution mixes with the oxygen-containing gas stream in the jet aeration nozzle. A higher amount of shear induces more reactions by the thermophilic organisms, thereby producing more heat. Lowering the biosolids flow rate and/or the gas flow rate results in less shear, which in turn induces less exothermic reaction by the microorganisms.
By maintaining an autothermal, truly aerobic treatment environment, numerous process advantages ensue as well as a better digested biosolids product. Objects of the present invention include significantly reducing the volatile solids in the biomass, reducing the total mass of biosolids and producing a stabilized material suitable for land disposal. Another objection of the present invention is to create and maintain a truly aerobic environment for the treatment of waste water biosolids. A truly aerobic biological process has sufficient oxygen present to support the living organisms, respiration rates and does not allow an anoxic condition to occur.
The THERMAER(trademark) Process which incorporates the present invention involves the surprisingly effective use of lower air flows and higher liquid flows. Counterintuitively, the use of a lower airflow can actually increase the amount of oxygen imparted into solution. It is believed that using a lower air flow process results in the injection of extremely fine bubbles into the treatment solution and higher surface renewal of the solution.
The present invention facilitates the treatment of biosolids in an autothermal process by removing a high percentage of water and increasing the organic concentration in a biosolids thickening process that precedes introduction of the biosolids into the treatment reactor. By thickening the biosolids, the volume of the biosolids solution may be significantly reduced, thereby enabling greater temperature control through the use of liquid flow rate.
The inventive process may be tailored to virtually any individual application. Different industrial plants have different product mixes with different sets of constituents. The complexity of the organic chemistry can vary from short chain molecules that are readily broken down to long chain molecules that are difficult to break down. The THERMAER(trademark) Process has the flexibility to deal with varying plant conditions and can operate at varying liquid depths, at varying hydraulic and solids retention times and operate as a single tank reactor or multiple tank reactors.
In the preferred embodiments of the present invention, the temperature of a truly autothermal aerobic process is controlled through a variable frequency drive on a jet motive pump which circulates biosolids through the jet aeration device into the reactor. Reactor temperature is controlled by varying the force in which the biosolids solution is circulated or re-circulated into the reactor through an aeration jet or other suitable means. In other embodiments, reactor temperature is controlled through the air pump used to control the flow rate of oxygen-containing gas through the jet aeration device.
In the present invention, the perceived problem of foaming caused by the treatment process is turned into an advantage. The inventor has noted that foam can act as an insulator between the biosolids solution and the air in the top of the reactor. In a typical reactor, the reactor is vented to the atmosphere so that it is not under pressure. As a result, the temperature of the air in the reactor is affected by the temperature of outside the reactor; in some cases, the temperature of the air in the reactor may be the same as the ambient temperature outside. By refraining from destroying all the foam bubbles, it is possible to use the foam as an insulator between the biosolids solution and the air in the reactor. Preferably, a foam control system is operated to maintain a layer of foam having a depth of from about four to about eight feet, preferably about six feet.
The inventive process may be used to treat a biosolids solution comprised of the products of waste water treatment and thermophilic bacteria capable of digesting mesophilic bacteria. The process comprises the steps of (a) thickening biosolids solution before it first enters a biosolids treatment reactor to a concentration of from about 3% to about 6% solids; (b) mixing a portion of biosolids solution with an oxygen-containing gas stream using a jet aeration device; (c) injecting a mixture of the oxygen-containing gas and biosolids solution into a reactor at a flow rate which introduces sufficient oxygen into the study solution so that the treatment environment is substantially constantly aerobic; and (d) controlling the temperature of the biosolids solution by adjusting an amount of shear generated through the jet aeration device. In some embodiments, the amount of shear (and the temperature of the biosolids solution) is controlled by adjusting the liquid flow rate of biosolids through the jet aeration device while keeping the flow rate of oxygen-containing gas constant. In most embodiments the portion of biosolids solution mixed with oxygen-containing gas in the jet aeration device will be recirculated biosolids that has been removed from the general biosolids solution in the reactor and pumped through the jet aeration device.
The inventive process may also include the step of wasting a portion of treated biosolids wherein the wasting step is performed in the same apparatus in which the thickening step is performed. xe2x80x9cWastingxe2x80x9d is a term used in the industry to mean dewatering biosolids prior to its disposal.
Alternate embodiments of the present invention comprise an apparatus for autothermal aerobic treatment of wastewater treatment biosolids. That apparatus comprises a means for concentrating a wastewater treatment biosolids to a concentration of at least about 3 percent solids. Among the suitable means for concentrating the biosolids solution are a horizontal solid bowl-decanting centrifuge, a gravity belt, a rotary drum thickener, dissolved air flotation, gravity settling, or the application of evaporative heat. The apparatus also comprises a reactor having an inlet from said concentrating means for the introduction of at least one biosolids and a jet aeration device affixed to the bottom of the reactor.
The jet aeration device comprises an air header having one or more openings through which a gas transported through the air header may exit the air header; a liquid header running parallel to and/or concentric with the air header and having one or more openings through which a biosolids solution transported through the liquid header may exit the liquid header; an outer nozzle extending from the liquid header and having an opening on its side; an inner nozzle from the liquid header and contained within the outer nozzle; one or more air passage connections from the air header to the outer nozzle which connects the air header to the liquid header and provides a closed path for air from the air header to travel to the outer nozzle and enter the outer nozzle through its side opening; and liquid from the liquid header are mixed in the outer nozzle. The apparatus comprises an air distribution pipe connected to the air header, which provides an oxygen-containing gas from outside the reactor; and a liquid outlet located at or near the bottom of the reactor, which allows biosolids solution to exit the reactor. The apparatus may optionally include a motive pump connected to the liquid outlet such that biosolids solution is withdrawn from the reactor by the motive pump. Attached to the motive pump is a motive pump conduit that leads from the motive pump to the liquid header such that biosolids solution is pumped through the conduit into the liquid header and forced through the inner nozzle by force of the motive pump.
The present invention may also include apparatus for automatically sensing and controlling the temperature in the reactor by adjusting the rate at which liquid is circulated into the reactor through the jet aeration device. This apparatus will typically include a temperature sensor within the reactor and means for automatically controlling the motive pump. Suitable means for automatically controlling include a programmable logic controller (xe2x80x9cPLCxe2x80x9d), a computer, analog signal or a microprocessor. This automatic control means is operatively attached to the temperature sensor and the motive pump such that based on the temperature of the biosolids solution in the reactor as measured by the temperature sensor, the automatic control means will instruct the motive pump to adjust the flow of biosolids solution through the liquid header in order to adjust the temperature of the biosolids solution in the reactor.
Apparatus embodying the present invention may also comprise a secondary cooling system, which comprises a cooling jet nozzle located in the reactor above the level of the jet aeration device; and a cooling conduit extending from the motive pump conduit to the cooling jet nozzle such that biosolids solution traveling through the cooling conduit loses heat to the surrounding environment.
In one embodiment of the present invention, the reactor holds a biosolids solution having a depth of at least about 24 feet. Another benefit of the present invention is it can be used in larger reactors. Because the invention can be used in larger reactors, the residence time of biosolids in a reactor can be increased so that biosolids may remain in a single reactor throughout the entire treatment period.
As discussed above, the foam created during the treatment process can be used to advantage, as an insulator between the biosolids solution and the air in the reactor. Nonetheless, a reliable foam control system is necessary to maintain a layer of foam at a desirable depth and prevent an excess of foam from escaping from the reactor.
In a further refinement of the THERMAER(trademark) Process, an inventive method and apparatus for foam control system has been developed. This method and apparatus may be used in conjunction with or separately from the other steps and apparatus of the THERMAER(trademark) Process described herein.
In one embodiment, the method comprises the additional or separate steps of generating a layer of foam on top of the biosolids solution, transferring a portion of the layer of foam from on top of the biosolids solution into the biosolids solution through a foam transfer pipe, and converting at least some of the portion of the layer of foam into liquid during transfer through the foam transfer pipe. The foam transfer pipe preferably includes a static mixer or other means that impart a dynamic mixing action to the foam, thereby rupturing or collapsing foam bubbles. Dynamic mixing action is action that imparts turbulence or energy that causes foam bubbles to collapse or rupture. One way to impart dynamic mixing action is to cause the fluid to have turbulent flow; another way is to mix the fluid or cause the fluid to move in a swirling motion. Alternately, the method may comprise the steps of transferring a portion of the foam from on top of the solution into the solution through the foam transfer pipe; mixing the foam in the foam transfer pipe so that at least some of the portion of foam is converted to liquid while passing through said foam transfer pipe; and drawing at least a portion of foam (which may be converted to liquid) by suction through at least a portion of the foam transfer pipe. The source of the suction may be an outer nozzle of a jet aeration system similar to those described herein, except that one outer nozzle is not connected to an air header; instead, it is dedicated to the foam transfer pipe. As fluid flows through the inner nozzle, it generates a vacuum or draw in the outer nozzle that pulls or sucks liquified foam from a foam transfer pipe that is fluidly connected to the side of the outer-nozzle.
The foam control apparatus is preferably used in connection with the ATAD treatment reactor comprising a jet aeration system as described above. The foam control apparatus comprises a foam transfer pipe having a top opening, a bottom opening and an internal surface, wherein said top opening is at least above an anticipated level of a solution (for example, a biosolids solution), the bottom opening is at least below the anticipated level of the solution and is fluidly connected to a suction source. The suction source is preferably an outer nozzle of a jet aeration device that is dedicated to the foam transfer pipe or another venturi device. The foam transfer pipe preferably has a static mixer disposed therein. The static mixer may be affixed to the internal surface of the foam transfer pipe.